Hello SGaist, I started lupdate from Qt Creator menubar->Tools->External->Linguist->Update Translations (lupdate).
It seems there is a bug in Qt Creator 4.3.1. With Qt 5.9 and Qt Creator 4.3.0 lupdate works successfully

hint: as described here I'm always changing the path to the executable and setting explicitely, because I had some issues with older QtCreator Versions and Android with lupdate / lrelease using the default values from Tools external